1. Technical Field
Aspects of the present invention relate to a communication system in which data communication is performed using electronic certificate.
2. Related Art
Communication systems configured to perform data communication retaining a high level of security by enabling a receiving device to check validity of information transmitted from a sending device in accordance with reliable information stored in advance in the receiving device have become widespread. An example of such a communication system is disclosed in Japanese Patent Provisional Publication No. 2005-141317A. Another example of such a communication system is configured such that data communication having a high level of security is achieved through use of an electronic certificate.
Recently, SSL (Secure Socket Layer) communication technology has been proposed as one of communication systems which use an electronic certificate. To achieve SSL data communication, a pair of a secret key and a public key is generated, and an electronic certificate for the public key is issued by a CA (Certification Authority) before the data communication is started. Then, the secret key and the electronic certificate of the public key are installed onto a server to be involved in the SSL data communication.
When an initial signal of an SSL handshake is transmitted form a client to the server on which the electronic certificate and the secret key are installed, the server transmits the electronic certificate to the client. Then, the client checks the validity of the electronic certificate transmitted from the server in accordance with a digital signature of the received electronic certificate and a CA certificate. That is, in this case, the validity of the electronic certificate is checked depending on the CA certificate which is an electronic certificate of the CA. When the client judges that the received electronic certificate is valid, the client performs data communication using a public key indicated by the electronic certificate received from the server so that confidential information can be transmitted to or received from the server.
To ensure data communication having a high level of security, it is desirable to refer issuance of electronic certificates to a third party serving as a CA. However, use of a third party as a CA increases cost of the communication system. For this reason, in a small network system, a single device capable of serving as a CA and a server is frequently adopted.